1. Field of the Invention
The present invention relates to a fresnel lens having a shading layer and a display device such as a liquid crystal display device including magnifying fresnel lenses.
2. Description of the Related Art
Liquid crystal display devices can have relatively thin structures and have been used for many applications. Recently, projection type liquid crystal display devices having larger screens have been developed. A typical projection type liquid crystal display device includes a projection lens which projects a magnified image onto a screen. Also, optical elements other than a projection lens can be used for magnifying an image.
For example, Japanese Unexamined Patent Publication (Kokai) No. 5-188340 discloses a projection type liquid crystal display device including liquid crystal display panels, fresnel lenses for magnifying images produced by liquid crystal display panels, and a screen. In this case, the liquid crystal display device also includes arrays of convergently transmissive elements, and a screen. Each of the arrays of convergently transmissive elements is adapted to form an erect and real image having an identical size to an object, and each of the fresnel lenses serves to magnify the image from the array of convergently transmissive elements.
The convergently transmissive elements are made from plastic or glass in the form of transparent rods having the diameter of 1 mm to 2 mm, so that refractive index changes in each of the transparent rods in the radial direction thereof. By appropriately selecting the length and the distribution of refractive index thereof, it is possible to use each of the convergently transmissive elements so that it can form an erect and real image having an identical size to an object. A plurality of convergently transmissive elements are arranged in a close relationship to each other with the end surfaces of the elements arranged in a line or in a plane, to thereby form a row or an array of convergently transmissive elements. The array of convergently transmissive elements can be used as an imaging device for producing an erect and real image having an identical size to an object. The imaging device using the array of convergently transmissive elements has advantages, compared with a usual spherical lens, in that a focal distance is very short and an optical performance is uniform in the line or plane so that an adjustment of the distance between the lenses is not necessary.
However, when the array of convergently transmissive elements is used as the imaging device, it is not possible to change a magnification of the image although it is possible for individual convergently transmissive elements to be changed in magnification by changing the length of the elements. This is because magnified images produced by the individual convergently transmissive elements are inconsistently superposed, one on another, in the array and a normal image cannot be formed. Therefore, the array of convergently transmissive elements can be used only as a full size imaging device, and it is necessary to provide a magnifying means in addition to the array of convergently transmissive elements.
Japanese Examined Patent publications (Kokoku) No. 58-33526 and No. 61-12249 disclose an imaging device including an array of convergently transmissive elements and a convex lens or a concave lens as a magnifying means which is arranged on the inlet side or on the outlet side of the array of convergently transmissive elements. The convex lens or the concave lens can be of a single lens or a composite lens of a plurality of lens components to realize a desired magnification. However, when this imaging device is used with a magnifying device in a liquid crystal display device, a problem arises in that resolving power of the lens changes from the central portion to the peripheral region.
It has been found that a good image is obtained if the resolving power MTF is greater than 50 percent under the condition of 4 (1 p/mm) i.e., 4 pairs of white and black spots per millimeter. However, it is generally difficult to establish an image having resolving power MTF greater than 50 percent in the above described prior art. It is necessary that light passes through the peripheral region of the liquid crystal display panel at an angle of approximately 10 degrees relative to the normal line of the liquid crystal display panel in order to ensure resolving power MTF greater than 50 percent. The smaller the angle at the peripheral region is, the smaller the magnification of the device is. As a result, it is not possible to realize a liquid crystal display device having a thin structure if a convex lens or a concave lens is used with an array of convergently transmissive elements, although the array of convergently transmissive elements by itself can provide a liquid crystal display device having a thin structure.
Accordingly, a magnifying element is desired which can be used with an array of convergently transmissive elements and which can realize a liquid crystal display device having a thin structure. The use of a fresnel lens with an array of convergently transmissive elements is disclosed in the above described Japanese Unexamined Patent Publication (Kokai) No. 5-188340, but the manner in which the fresnel lens is used is not described in this prior art. The inventors have recently found that a good result is obtained if a fresnel lens is used as a magnifying element.
Further, in a liquid crystal display device, there is a problem that brightness of an image on a peripheral region of the screen is reduced relative to the brightness of the image on the central region of the screen.
The object of the present invention is to provide a fresnel lens constructed such that light is made incident to a configured surface thereof.
Another object of the present invention is to provide a display device having a thin structure by appropriately arranging a fresnel lens.
Another object of the present invention is to provide a display device in which the brightness of a screen is improved.
According to one aspect of the present invention, there is provided a fresnel lens comprising a body having a flat surface and a configured surface with periodic ridges, each of the ridges including a flat crest extending generally parallel to the flat surface and at least one inclined surface extending from the flat crest toward the flat surface, and a shading layer provided on the flat crest of each of the ridges.
Preferably, the flat crests have varying widths depending on the positions of the ridges. In this case, the at least one inclined surface comprises a main inclined surface arranged on one side of the flat crest and designed such that light is mainly incident to the body from the main inclined surface and a minor inclined surface arranged on the other side of the flat crest from the main inclined surface.
Preferably, the width of the flat crest is determined by the following relationship:                     d        =                  p          ⁢                      xe2x80x83                    ⁢                                    tan              ⁢                              xe2x80x83                            ⁢              r                                                      tan                ⁡                                  (                                      90                    -                                          θ                      ⁢                                              xe2x80x83                                            ⁢                      1                                                        )                                            +                              tan                ⁢                                  xe2x80x83                                ⁢                r                                              xc3x97                      [                          1              -                                                tan                  ⁢                                      xe2x80x83                                    ⁢                  θ                  ⁢                                      xe2x80x83                                    ⁢                  2                                                  tan                  ⁢                                      xe2x80x83                                    ⁢                  r                                                      ]                                              (        1        )            
where d is the width of the flat crest, p is the pitch of the ridges, r is the angle of a major light ray made incident to the body from the main inclined surface relative to the axis, xcex81 is the angle the main inclined surface relative to the flat surface, and xcex82 is the angle of the minor inclined surface relative to the axis.
According to a further aspect of the present invention, there is provided a display device comprising at least one image modulator, an array of convergently transmissive elements receiving light from said at least one image modulator for forming an erect and real image, a fresnel lens including a body having a flat surface and a configured surface with periodic ridges, the fresnel lens being arranged so that light is made incident from the array of convergently transmissive elements to the configured surface of the fresnel lens, and a screen receiving light from said at least one image modulator via the array of convergently transmissive elements and the fresnel lens.
Preferably, each of the ridges includes a flat crest extending generally parallel to the flat surface and at least one inclined surface extending from the flat crest toward the flat surface, and a shading layer is provided on the flat crest of each of the ridges.
Preferably, the flat crests have varying widths depending on the positions of the ridges. Preferably, the at least one inclined surface comprises a main inclined surface arranged on one side of the flat crest and designed such that light is mainly incident to the body from the main inclined surface and a minor inclined surface arranged on the other side of the flat crest from the main inclined surface.
Preferably, the at least one image modulator comprises a plurality of liquid crystal display panels, and the array of convergently transmissive elements and the fresnel lens are arranged for every liquid crystal display panel. Preferably, four sets of the liquid crystal display panels, the arrays of convergently transmissive elements and the fresnel lenses are arranged, with each set arranged in respective quarter portions in a rectangular region, the screen having a total display area four times greater than a display area necessary to receive an image from one set of the liquid crystal display panel, the array of convergently transmissive elements and the fresnel lens.
Preferably, a partition is arranged on or near the screen between two adjacent sets of the liquid crystal display panels, the arrays of convergently transmissive elements and the fresnel lenses for preventing light from straying from one set into the adjacent set.
Preferably, the screen has a predetermined display area, and said at least one image modulator has a main display area and a peripheral compensating area arranged such that the main display area forms an image on the predetermined display area via the array of convergently transmissive elements and the fresnel lens and the peripheral compensating area forms an image just outside the predetermined display area via the array of convergently transmissive elements and the fresnel lens. Preferably, the peripheral compensating area of said at least one image modulator is controlled to provide an image which is generally identical to a portion of an image delivered from the main display area of the at least one image modulator near the peripheral compensating area.
Preferably, between two adjacent liquid crystal display panels, said peripheral compensating area of one liquid crystal display panel is controlled to provide an image which is generally identical to a portion of an image delivered from the main display area of the adjacent liquid crystal display panel near the peripheral compensating area of said one liquid crystal display panel.
According to a further aspect of the present invention, there is Provided a display device comprising at least one image modulator, optical lens for magnifying an image output by said at least one image modulator, a screen for receiving an image from said at least one image modulator via said optical lens, the screen having a predetermined display area, and said at least one image modulator has a main display area and a peripheral compensating area arranged such that the main display area forms an image on the predetermined display area via said optical lens and the peripheral compensating area forms an image just outside the predetermined display area via said optical lens.